1. Field of the Invention
The present invention concerns an improvement to a driver for semiconductor lasers used in optical data processing and fiber optic transmission.
2. Prior Art
Because of the high optical output, small size and light weight, the practical application of semiconductor lasers is rapidly spreading as a light source for optical data processing and fiber optic transmission systems. Single mode semiconductor lasers with a stabilized transverse mode have been developed for such purposes. However, when the single transverse mode laser is used with such equipment or systems, mode hopping noises are induced in the laser by a substantial reflection of light from the various components of the system, thereby presenting grave problems in practical application.
As is shown in Ohishi et al's paper "Lowering of noises in semiconductor laser by high frequency superposition and longitudinal mode characteristics" published in Collection of Proposed Lectures for the 44th Conference of the Society of Applied Physics, page 102, an external oscillator is employed to apply high frequency current near the relaxation oscillation frequency of the semiconductor laser. A driver is employed for matching the frequency to the single transverse mode semiconductor laser in order to stably maintain the longitudinal multimode oscillation and to lower the coherence. However, the adoption of such a driver not only renders the laser device complex and expensive, but also requires the adjustment of the oscillating frequency, current amplitude and DC bias current of the external oscillator for every semiconductor laser used. This necessary adjustment comes from the fact that the noise characteristics vary greatly depending on the structural parameters and operational conditions of each semiconductor laser.